It was recalled that photo-reductive dissolution of layer-type MnIV oxides

(birnessite) in sunlight, to form soluble MnII was observed in both field and

laboratory; leading to the consensus that the process was a key factor in the biogeochemical

cycling of Mn. However, the underlying mechanisms for the process

remained unknown, although they were linked with the semiconducting

characteristics of hexagonal birnessite. One of its universal properties was the

presence of MnIV vacancies: long-identified as being strong adsorption sites for

metal cations. Here, the possible role of Mn vacancies in photo-reductive

dissolution was investigated theoretically using quantum-mechanical calculations

based upon spin-polarized density functional theory. The study proved that Mn

vacancies significantly reduced the band-gap energy for hexagonal birnessite

relative to a hypothetical vacancy-free MnO2, and would thus increase the

concentration of photo-induced electrons available for MnIV reduction upon

illumination of the mineral by sunlight. Calculations of the charge distribution in

the presence of vacancies, although not fully conclusive, indicated a clear

separation of photo-induced electrons and holes; implying a slow recombination of

these charge-carriers that facilitated the two-electron reduction of MnIV to MnII.

On the Role of Mn(IV) Vacancies in the Photoreductive Dissolution of Hexagonal

Birnessite. K.D.Kwon, K.Refson, G.Sposito: Geochimica et Cosmochimica Acta,

2009, 73[14], 4142-50